The present invention relates generally to the field of measuring devices, and more particularly, is directed to a system for remotely measuring the level of contained fluids and other media.
Many types of level sensors are known in the prior art for remotely measuring the level of fluids, powders and granular material contained in tanks and bins. The sophistication and accuracy of these devices vary with the requirements of the application. Non-contact level sensing instruments using ultrasonic or radiation detectors are among the most sophisticated and accurate measuring devices in use today. Because of their complexity and cost, however, these devices are restricted to industrial and commercial applications. Designers of level sensors for consumer applications, such as for use in automobiles and trucks, have traditionally placed the greatest effort on keeping cost low. Thus, most such sensors are rudimentary float based devices which produce an electrical signal proportional to the fluid level being measured. Such devices have not proved reliable for measuring the level of fluids in non-stationary containers such as the oil reservoir and fuel tank in an automobile or truck. More sophisticated level sensors using a capacitance principle have been used in aircraft and experimentally evaluated in automotive applications. These designs were complicated, fairly expensive, required special wiring and could not endure the operating environment imposed by vehicular use.
Capacitive level sensors known in the prior art utilized frequency sensitive circuitry wherein a change in sensor capacitance caused by a change in the dielectric constant of the surrounding medium produces a change in the frequency of a high frequency oscillator. It is well established that oscillator frequency varies as a function of f=(2n.sqroot.LC).sup.-1 for an LC (inductance-capacitance) oscillator and that time constant t changes for an RC (resistance-capacitance) oscillator. The changes in f or t are minimal for small changes in "C". Sensitivity of the oscillator frequency to environmental changes (humidity, temperature, vibration, etc.) must necessarily be kept low if the typically small changes in sensor capacitance are to be measured with any precision. Another problem with prior art capacitive level sensors, and perhaps the most serious, is the requirement to measure very small changes in capacitance. These changes are typically on the order of 10 pF or less for reasonably sized sensors. It is known that the capacitance of a multiplate capacitor is given as follows: EQU CpF=0.0885.times.KS(N-1)d.sup.-1
where
K=dielectic constant PA1 S=area of each plate in cm.sup.2 PA1 N=number of plates PA1 d=thickness of dielectric in cm.
Unless a large number of parallel plates are used, the actual capacitance value of a sensor of practical size is relatively low. This is especially true when the inter-plate spacing of one the order of several millimeters. Minimal plate spacing of one mm or more may be required, however, because, as discovered in various experiments, the surface tension and viscosity of some fluids (motor oil for example) induces capillary attraction between the capacitor plates, thereby causing erroneous readings. The ability of the sensor to shed fluid after submersion and subsequent removal from the fluid is an important consideration in many applications. This property is also a function of surface tension and viscosity. Low capacitance sensors with relatively wide inter-plate spacing assure proper fluid drain-down and minimizes capillary "wicking".
In measuring the small change in sensor capacitance due to the presence or absence of surrounding fluid (or other media), the added shunt capacitance of the sensor leads is an important consideration as well. If the shunt capacitance of the leads constitutes an appreciable fraction of the sensor net capacitance, the signal to noise ratio of the system deteriorates, i.e., the minimum resolution of the sensor diminishes.
U.S. Pat. No. 4,214,479 to Maier discloses a capacitive type sensor used to measure the mass of fuel in a fuel tank. The probe is connected to a source of power for supplying current to the probe capacitor and is immersed in the mass of fuel. The probe capacitor current is summed with a current corresponding to the capacitance of the fuel tank in the empty state to provide an output voltage. The output voltage is positively integrated to a predetermined value as determined by the number of pulses counted by a counter. The count is then actuated to count down the number of pulses required to return the integrated output voltage to zero. The number of pulses counted down provides a digital representation proportional to the mass of fuel sensed by the probe capacitor.
Also known in the prior art are a number of sensor probe configurations used with measuring systems of various types. For example, U.S. Pat. No. 4,329,644 to Libertini et al. discloses a high temperature probe for detecting shaft or rotor speed in a gas turbine engine. As shown in FIG. 4, the probe comprises a housing 2 which receives electrode assemblies 4 and 5. Each electrode assembly is surrounded by air gap 32 and is held in position by a pair of insulating rings 8 and 9 formed of synthetic sapphire. The other structural elements in the probe are formed from KOVAR.
U.S. Pat. No. 4,314,428 to Beaman discloses a capacitance probe comprising at least two parallel blades. The blades are generally rectangular in construction and are of relatively large surface area.
U.S. Pat. No. 3,918,306 to Maltby discloses a system for measuring vehicle transmission fluid based on the capacitance between a pair of probes. The probes are coupled to an impedance network. In order to compensate for changes in transmission fluid level due to changes in temperature, the network includes a temperature sensitive impedance.
British Patent No. 989,618 discloses a liquid measurement device which has a first probe formed of a continuous length of wire disposed in a plurality of parallel paths extending lengthwise within a cylindrical structure. The cylindrical structure forms the second electrode.
While the above described measuring systems known in the prior art perhaps represent an improvement over older such systems, they remain deficient in a number of areas. For example, the Maier level measuring device requires a number of component parts to operate and cannot be used in high temperature environments. At low temperature, many fluids, such as oil for example, are highly viscous and tend to adhere to adjacent parts. Thus, in a level measuring system that uses a solid cylinder such as disclosed in the above cited British patent, the oil collects on the surface and is difficult to shed. If the oil is not readily shed, the sensor provides a false reading. Moreover, the parallel probe element must be positioned close to the cylinder in order to get a measurable change in capacitance as the fluid level changes. The closer the parallel probe element is to the cylinder, however, the less likely the oil will shed from the sensor and provide an accurate reading. Also, in probes of the type disclosed in the Beaman patent, it is difficult to align the probe blades with respect to the fluid surface. Such probes must be aligned perpendicular to the surface of the fluid in order to achieve maximum sensitivity. This is often difficult to do because the probe tip is not always visible during installations and use. Accordingly, there is an unfilled need in the art for a fluid level measuring system which is reliable in operation, easy to install and low in cost.